refactor(scan): validate FileScanTask construction

[blackmwk]

Which issue does this PR close?

• Closes Remove the public field accessor of FileScanTask #3130.

What changes are included in this PR?

• Make FileScanTask fields private and expose read-only public getters.
• Make FileScanTaskBuilder::build() validate its result without depending on generated typestate arguments.
• Validate symmetric partition/spec presence and partition tuple arity.
• Validate partition specs against the task schema by calling PartitionSpec::partition_type directly.
• Keep task construction lightweight by avoiding per-value partition validation.
• Update internal call sites for the fallible builder and regenerate the public API snapshot.

Are these changes tested?

• Added builder regression coverage for non-empty partitions without specs, partitioned specs without values, valid unpartitioned forms, arity mismatches, incompatible evolved schemas, and missing partition source columns.
• Updated scan and metadata-only Arrow fixtures for the validated construction rules.
• Ported the construction checks from feat(scan): make FileScanTask serializable #3091 while following review direction to omit per-value validation from this path.
• cargo test -p iceberg --lib (1,623 passed)
• cargo clippy --workspace --all-targets -- -D warnings
• make check-public-api
• cargo fmt --all -- --check

AI Disclosure

Codex (GPT-5) was used for implementation, call-site refactoring, review follow-up, and unit-test scaffolding. The resulting diff was reviewed against issue #3130, the relevant checks in PR #3091, the latest inline reviews, and repository conventions. All listed checks were run locally; there are no known uncertainties.

[CTTY]

LGTM!